Problem: The scale on a map is 10cm : 10km. If the distance between two cities is 20km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 10cm on the map corresponds to an actual distance of 10km. An actual distance of 20km is the same as 2 $\cdot$ 10km. The distance between the two cities on the map is 2 $\cdot$ 10cm, or 20cm.